Theoretical astrophysics and plasma physics at RPC

Abstract:

This is the first of a series of papers dedicated to unveiling the mass composition and dynamical structure of a sample of flattened early-type galaxies in the Coma cluster. We describe our modifications to the Schwarzschild code of Richstone et al. Applying a Voronoi tessellation in the surface of section, we are able to assign accurate phase-space volumes to individual orbits and to reconstruct the full three-integral phase-space distribution function (DF) of any axisymmetric orbit library. Two types of tests have been performed to check the accuracy with which DFs can be represented by appropriate orbit libraries. First, by mapping DFs of spherical γ-models and flattened Plummer models onto the library, we show that the resulting line-of-sight velocity distributions and internal velocity moments of the library match those derived directly from the DF to a precision better than that of present-day observational errors. Secondly, by fitting libraries to the projected kinematics of the same DFs, we show that the DF reconstructed from the fitted library matches the input DF to a rms of about 15 per cent over a region in phase space covering 90 per cent of the mass of the library. The accuracy achieved allows us to implement effective entropy-based regularization to fit real, noisy and spatially incomplete data.

Abstract:

[Abridged] We present a novel technique, dubbed FiEstAS, to estimate the underlying density field from a discrete set of sample points in an arbitrary multidimensional space. FiEstAS assigns a volume to each point by means of a binary tree. Density is then computed by integrating over an adaptive kernel. As a first test, we construct several Monte Carlo realizations of a Hernquist profile and recover the particle density in both real and phase space. At a given point, Poisson noise causes the unsmoothed estimates to fluctuate by a factor ~2 regardless of the number of particles. This spread can be reduced to about 1 dex (~26 per cent) by our smoothing procedure. [...] We conclude that our algorithm accurately measure the phase-space density up to the limit where discreteness effects render the simulation itself unreliable. Computationally, FiEstAS is orders of magnitude faster than the method based on Delaunay tessellation that Arad et al. employed, making it practicable to recover smoothed density estimates for sets of 10^9 points in 6 dimensions.

Abstract:

We present surface photometry for the central regions of a sample of 48 spiral galaxies (mostly unbarred and barred of type Sbc or Sc) observed with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. Surface brightness profiles (SBPs) were derived and modeled with a Nuker law. We also analyzed archival Wide Field Planetary Camera 2 images with a larger field of view, which are available for 18 galaxies in our sample. We modeled the extracted bulge SBPs with an exponential, an r1/4, or an r n profile. In agreement with previous studies, we find that bulges of Sbc galaxies fall into two categories: bulges well described by an exponential profile and those well described by an r1/4 profile. Only one galaxy requires the use of a more general Sérsic profile to properly describe the bulge. Nuclear photometrically distinct components are found in ∼55% of the galaxies. For those that we classify as star clusters on the basis of their resolved extent, we find absolute magnitudes that are brighter on average than those previously identified in spiral galaxies. This might be due to a bias in our sample toward star-forming galaxies, combined with a trend for star-forming galaxies to host brighter central clusters.